The present invention relates to a color signal network system for performing conversions on color signals.
As example of the apparatus for outputting color signals that has known prior art device for performing color signal conversions is described in Unexamined Published Japanese Patent Application No. 145770/1985. FIG. 9 shows a layout for the apparatus proposed in that patent which is used to prepare a printing color-separated plate. As shown, a color document 101 is wound onto a scan drum 102 and the image on the document is read with a scanning mechanism 103. Three color signals R, G and B obtained by color separation are sent to a logarithmic converting stage 105 where they are converted to complete or partial logarithmic colorimetric signals R', G' and B'. The signals stage 105 are supplied into the first correction circuit 106 where color-separated plate signals C, M and Y for subtractive mixing and, optionally, K are generated. These color-separated plate signals C, M and Y are fed into a superposing stage 107 where selective correction signals Ck, Mk and Yk are superposed on C, M and Y. The resulting superposed signals C', M' and Y' are supplied into a recording mechanism 108 for performing color recording on associated recording media 110 that are wound onto recording drums 109.
The apparatus for preparing a printing color-separated plate that is proposed in Unexamined Published Japanese Patent Application No. 145770/1985 is equipped with a color signal output device 112. This color signal output device 112 is supplied with the three color signals R, G and B obtained from the scanning mechanism 103. The input signals are converted to complete or partial logarithms in a converting stage 113 to generate colorimetric signals R', G' and B'. These colorimetric signals R', G' and B' are subjected to matrix operations by the following Eq. (1) and converted to chromaticity signals x, y and lightness signals z (luminance/chromaticity separated signals): EQU x=a.sub.11 R'+a.sub.12 G'+a.sub.13 B' EQU y=a.sub.21 R'+a.sub.22 G'+a.sub.23 B' EQU z=a.sub.31 R'+a.sub.32 G'+a.sub.33 B' (1)
In the apparatus under discussion, the signal output device 112 uses those luminance/chromaticity separated signals to compute the amounts of adjustment of hue, saturation and lightness and generates color control signals associated with these attributes. Based on the resulting color control signals, the signal output device 112 calculates the aforementioned selective correction signals Ck, Mk and Yk, which are fed into the superposing stage 107 and respectively added to output signals C, M and Y from the first correction circuit 106.
As described above, the three color signals R, G and B obtained by color separation in the apparatus under discussion are converted to luminance/chromaticity separated signals by the combination of conversions to complete (or partial) logarithms and matrix operations. Therefore, conversion to correct color signals (colorimetric values) is impossible unless the characteristics of separation into three colors are color matching functions. If, in this situation, one wants to supply color signals into a network, accurate color reproduction is not likely to occur at the receiver's end.
Another disadvantage of the apparatus is that although it is capable of conversion to a certain type of luminance/chromaticity separated signals, conversions to various types of color signals (e.g. CIE XYZ and NTSC YIQ) are impossible unless it has a means of rewriting both the contents of the logarithmic (or partial logarithmic) converting states and the coefficients of matrices. This presents a problem if the receiver accepts only different types of color signals than those which are sent out from the apparatus.
If the characteristics of separation into three colors are color matching functions and if the logarithmic (or partial logarithmic) converting stage is composed of one-dimensional look-up tables (hereunder abbreviated as LUTs), the availability of the means of rewriting the contents of LUTs and the coefficients of matrices enables conversions to several accurate luminance/chromaticity separated signals using the LUTs and performing matrix computations. A few comments here on this point may be in order.
If the characteristics of separation into three colors are color matching functions, the three color signals R, G and B obtained by color separation are essentially colorimetric values. However, nonlinear sensors or circuits must be corrected (scanner calibrated) and one-dimensional LUTs are used to perform this correction. It is known that the essentially colorimetric three signals R, G and B can be accurately converted to CIE XYZ signals by matrix computations of the same form as Eq. (1). In other words, R, G and B can be converted to correct chromaticity signals X and Z and luminance signal Y. When the receiver is a CRT, the one-dimensional LUTs may be adapted to perform simultaneously the scanner calibration and the gamma correction of the image-receiving tube of the CRT. In this case, the one-dimensional LUTs will output gamma-corrected colorimetric values RGB. These RGB signals can also be converted to gamma-corrected colorimetric values NTSC YIQ by matrix computations of the same form as Eq. (1). In other words, conversion to correct chromaticity signals I and Q and luminance signal Y can be accomplished. Further, if the matrices are set as unit matrices, the colorimetric signals R, G and B can be output unaltered (without gamma correction).
As described above, if the characteristics of separation into three colors are color matching functions and if one-dimensional LUTs, a means of computing matrices and a means of rewriting the contents of the LUTs and the coefficients of matrices are available, conversion can be made not only to RGB signals (which may be gamma-corrected) but also to various correct luminance/chromaticity separated signals (which may also be gamma-corrected) that are expressed by linear transformations of RGB signals. Even in this case, however, accurate conversion to CIE LAB signals cannot be achieved for the following reasons.
CIE LAB (L*a*b*) signals are calculated from CIE XYZ signals by the following Eq. (2): ##STR1## Note that solving Eq. (2) inversely for f(X/X.sub.0), f(Y/Y.sub.0) and f(Z/Z.sub.0) gives EQU f(X/X.sub.0)=a*/500+L*/116 EQU f(Y/Y.sub.0)=L*/116 EQU f(Z/Z.sub.0)=L*/116-b*/200 (2)
Hence, in order to accomplish accurate conversion from colorimetric RGB signals to CIE LAB signals, the following three blocks are necessary: first, a means of computing 3.times.3 matrices for obtaining CIE XYZ signals; second, one-dimensional LUTs for obtaining f(X/X.sub.0), f(Y/Y.sub.0) and f(Z/Z.sub.0) singles; third, a means of computing 3.times.3 matrices for obtaining CIE LAB signals from those f(X/X.sub.0), f(Y/Y.sub.0) and f(Z/Z.sub.0) signals. In other words, the combination of one-dimensional LUTs a matrix computing means and a means of rewriting both the contents of the LUTs and the coefficients of matrices is insufficient to achieve accurate conversion to CIE LAB signals.
Consider here a network to which a plurality of senders and receivers of color signals are connected. Various senders send out various color signals and each receiver must decode those signals accurately so that they are converted to its own internal color signals (CMY signals if the receiver is a printer). An example of the recording apparatus that is furnished with a known prior art device for performing this conversion of color signals is described in Unexamined Published Japanese Patent Application NO. 238937/1989. FIG. 10 shows a layout for the color image processing apparatus proposed in that patent. In this apparatus, three color separation signals obtained from a reader unit 216 and color signals supplied from a connected device 217 are converted to the same NTSC RGB signals, which are subjected to the steps of black generation and color correction to obtain a print using the CMYK color model. The operation of the apparatus is described in a little more specific way. The reader 216 reads a color document of interest and produces three color separation signals R, G and B. These signals are fed to a color converter circuit 206 where they are converted to NTSC RGB signals by matrix computations using the coefficients restored in a memory 209. In the present discussion, the connected device 217 is assumed to produce NTSC YIQ signals. These signals are also fed to the color converter circuit 206 where they are converted to NTSC RGB signals by matrix computations using the coefficients restored in a memory 210. The matrix computations for achieving this conversion to NTSC RGB signals are represented by the following Eq. (3): EQU Rn=1.00Y+0.96I+0.63Q EQU Gn=1.00Y-0.28I-0.64Q EQU Bn=1.00Y-1.11I+1.72Q (3)
Note that solving Eq. (3) inversely for YIQ gives EQU Y=0.30Rn+0.59Gn+0.11Bn EQU I=0.60Rn-0.28Gn-0.32Bn EQU Q=0.21Rn-0.52Gn+0.31Bn (3)
Unexamined Published Japanese Patent Application No. 238937/1989 discloses not only the use of a 3.times.3 matrix; it also teaches the use of matrices containing higher-order terms in order to accomplish more accurate color signal conversions.
However, the apparatus proposed in that patent does not have one-dimensional LUTs and if it is supplied with gamma-corrected NTSC YIQ signals, the apparatus is incapable of conversion to accurate (not gamma-corrected) NTSC RGB signals. To deal with this problem, the conversion curves in a density converter circuit 211 may be modified in such a way as to include a capability for dismissing the gamma correction. In fact, however, the apparatus is not provided with a means of altering the forms of those conversion curves.
As a further problem, the apparatus also does not take CIE LAB signals into account and even matrices containing higher-order terms are incapable of achieving accurate conversion from CIE LAB signals to NTSC RGB signals. The reason for this is as follows: CIE LAB signals are computed from CIE XYZ signals by Eq. (2), so in order to achieve accurate conversion from CIE LAB signals to NTSC RGB signals, three blocks, namely, a 3.times.3 matrix computing means for obtaining f(X/X.sub.0), f(Y/Y.sub.0) and f(Z/Z.sub.0) signals, one-dimensional LUTs for obtaining CIE XYZ signals, and a 3.times.3 matrix computing means for obtaining NTSC RGB signals from the CIE XYZ signals, are necessary. In other words, accurate conversion to CIE LAB signals cannot be accomplished merely by substituting matrices containing higher-order terms for the conversions achieved by those three blokes.
As described above in detail, color signal outputting apparatus and color recording apparatus that are equipped with conventional color converting devices have been unable to achieve accurate conversion from the internal color signals of those apparatus to various color signals (RGB signals, various luminance/chromaticity separated signals that are expressed by linear transformations of RGB signals, and CIE LAB signals, which signals may or may not be gamma-corrected) and vice versa.